The present invention relates to an improved arrangement for compensating the unwanted echoes, caused by the movement of a moving radar device.
Generally speaking, the arrangement for compensating the unwanted echo caused by a moving radar device, comprises a mixing device provided between the output of a coherent oscillator and the input of a phase discriminator. The mixing device comprises a series circuit composed of a first mixer, a first filter, a second mixer and a second filter. There is also provided a signal processing stage containing two series-connected mixing circuits, each of which mixing circuits delivers a signal, the frequency difference of which corresponds to the value of the Doppler frequency of the unwanted echo caused by the movement of the moving radar device. The first signal of the mixing circuits is supplied to the first mixer and the second signal of the mixing circuits to the second mixer of the mixing device. The frequency of the output signal of the first mixing circuit is formed by addition of the Doppler frequency of the unwanted echo caused by the movement of the moving radar device.
When using radar devices in mobile equipment there of course occur the aforementioned unwanted echos which must then be compensated when the inherent speed of the antenna is not negligibly small in comparison to that of a target.
From German Pat. No. 2,133,395 a Doppler radar device with fixed target suppression or cancellation is already known, wherein an arrangement to compensate the own movement of the transmitting/receiving antenna is provided between the output of the coherent oscillator and the input of a phase discriminator. This compensation arrangement consists of a signal processing stage and a mixing device, which is formed by the series circuit of a first mixer, a first filter, a second mixer and a second filter. The signal processing stage has a first mixing circuit, the output of which supplies a mixed signal which is fed to the input of a second mixing circuit, the output of which is connected to a further input of the second mixer of the mixing device. The frequency of this mixed signal corresponds to the sum of the coherent oscillator frequency, the self-movement Doppler frequency and an additional displacement frequency, which for reasons of symmetry is chosen at about Fr/4, Fr being the repetition frequency of the emitted pulses. The first mixing circuit delivers a further output signal, which is supplied to a further input of the first mixer of the mixing device.
The arrangement known from the aforementioned German Pat. No. 2,133,395 operates in such a manner that the signals reflected by targets are compared with regard to phase with the output signal of a coherent oscillator, and only those signals, the phase of which changes in relation to the coherent oscillator during two successive rotations of the antenna, are recognized as signals of moving targets, evaluated and indicated. In order to compensate the own movement, the relative velocity of the transmitting/receiving antenna to a stationary target is ascertained, and the frequency of the coherent oscillator is adjusted by the frequency of the Doppler shift or displacement belonging to this relative velocity, and the relative velocity is measured to a stationary target which is less far removed from the transmitting/receiving antenna than the target to be located.